There is technology of transmitting a signal transmitted using digital hierarchy technology such as PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical NETwork) (hereinafter referred to as “digital hierarchy signal”), via a packet network. When the digital hierarchy signal is input to the packet network, the digital hierarchy signal is converted into frames and transmitted as individual frames in the packet network. Also, when the individual frames are output from the packet network, the frames are converted into the digital hierarchy signal through signal extraction.
Further, in a conventional packet network, error detection is performed in each relay device. When a single transmission error is detected from a frame, the frame is discarded. This is because the packet network is designed for a computer and is intended to prevent a band from being unnecessarily consumed by immediately discarding the frame containing the error. However, when data requiring a real-time property is a transmission target, the data may need to be directly transmitted to a destination without retransmission even in a state in which an error is contained. For example, technology of directly transmitting data even when a bit error is generated, as shown in Patent Document 1, has been proposed.
FIG. 7 is a schematic diagram showing a problem generated when a digital hierarchy signal is transmitted via a packet network. In FIG. 7, a PDH signal is illustrated as a concrete example of the digital hierarchy signal. When the PDH signal is input to the packet network, the PDH signal is converted into a frame, and a plurality of frames are generated. In an example of FIG. 7, a 1-bit error is generated in one of the plurality of frames in transmission in the packet network. In this case, when error detection such as FCS (Frame Check Sequence) is performed in the packet network and an error is detected, a frame from which the error has been detected is discarded. Accordingly, the entire PDH signal contained in the discarded frame is discarded and, for example, a burst error of hundreds of bits or more is generated.